This invention relates generally to a method for thermal energy storage and, particularly, to a method for storing thermal energy using stratified chilled water.
There are a number of air conditioning and process cooling applications in which it is necessary to provide a measure of additional cooling capacity during peak cooling periods. It is also generally necessary to provide for the rejection of heat during these peak cooling periods. These requirements can be met using thermal energy storage.
One application for thermal energy storage might be found in a district cooling system for providing cooling for a large number of buildings from a single source. It is known in such a district cooling system that it is possible to utilize equipment that has less cooling capacity than the peak demand requires by using thermal energy storage. More specifically, the chilling equipment is operated at night during a minimal demand period and chilled water is stored in a large thermal storage tank. Then, when the demand increases during the day the chilled water is drawn off from the tank to improve the ability of the cooling equipment to provide the required cooling. As an example of the scale of such a district cooling system it is possible to use cooling equipment that has a 6,000 ton capacity to meet a cooling requirement of 11,000 tons by using a thermal storage tank holding around three million gallons of water.
Generally, such a thermal storage tank is always full, so that as cooled water is drawn off from the bottom of the tank the warmer water is returned at the top. The range of temperatures in such a tank is typically from 5.5.degree. C. (42.degree. F.) to 15.55.degree. C. (60.degree. F.) and between these temperatures the respective specific gravities of water increases steadily as the temperature drops resulting in gravity separation such as there would be in a conventional hot water tank, for example. Such thermal storage tanks usually have some sort of non-mixing inlet/outlet system so that the cooled outlet water is not mixed with the warm inlet water. Thus, the thermal storage tank will typically have a layer of chilled water at around 5.5.degree. C. (42.degree. F.) at the bottom with a layer of warm water of up to 15.55.degree. C. (60.degree. F.) on top of the chilled water.
Further economic savings and other advantages could be achieved if the temperature of the chilled water could be reduced without decreasing its density. A storage and pipeline system could double its capacity using the same volume of water and the system would be able to serve newer buildings which have 1.11.degree. C. (34.degree. F.) air conditioning systems. This is not possible in a system employing only water as the thermal mass sink, because the maximum density of water occurs at 4.0.degree. C. (39.2.degree. F.). At temperatures below 4.0.degree. C. (39.2.degree. F.) the density starts to decrease so the cooler water will rise and stratified chilled water cannot be maintained because the water mixes and the stratification is destroyed.
One common approach to increasing the ability of the thermal mass sink to accept the rejected heat is to use chilled or refrigerated aqueous solutions of brines, such as calcium chloride, sodium chloride, or glycol, all of which are capable of operating at temperatures below the freezing point of water, 0.degree. C. (32.degree. F.). While these brine solutions or glycol solutions have been used for many years, they each have particular problems which require either expensive or presently unacceptable remedies. For example, brine solutions are corrosive and require the use of a corrosion inhibitor. The most commonly used corrosion inhibitor has been sodium chromate. Today, however, sodium chromate is an environmentally unacceptable chemical and is, therefore, not available for use. On the other hand, industrial ethylene glycol solutions are normally used in the range of 25% and in addition to the prohibitive cost in installations of the size mentioned above also require inhibitors to control corrosion and chemical decomposition. Furthermore, microbiological decomposition of ethylene glycol can occur at solution concentrations below 20%, so that the higher solution concentrations must be used.